Reducing crown corrosion in a glassmelting furnace

ABSTRACT

Corrosion of the inner surface of the crown of a glassmelting furnace is reduced or avoided by directing at low velocity along that surface a gaseous stream comprising water vapor, or comprising the combustion products of an oxy-fuel burner operated at a stoichiometric ratio of at least 1.0, or the combustion products of a burner operated at a stoichiometric ratio less than 1.0, or by injecting into the furnace interior a gaseous reactant which reacts with alkali species in said space.

RELATED APPLICATIONS

This application is a divisional of prior U.S. application Ser. No.11/938,829, filed Nov. 13, 2007, and claims priority from, and alsoclaims priority from U.S. Provisional Application Ser. No. 60/859,783,filed Nov. 17, 2006, Ser. No. 60/859,784, filed Nov. 17, 2006, Ser. No.60/859,785, filed Nov. 17, 2006 and 60/859,786, filed Nov. 17, 2006.

FIELD OF THE INVENTION

The present invention relates to glassmaking and more particularly tothe furnaces that are employed in glassmaking.

BACKGROUND OF THE INVENTION

In the manufacture of glass, glassmaking materials are melted in aglassmelting furnace by heat provided from burners which combust fuelwith oxygen. The fuel can be combusted with air as the source of theoxygen, or with a stream containing a higher oxygen content than that ofair. The furnace must be manufactured of material that can withstand thevery high temperatures that prevail within the furnace. The materials ofconstruction often employed, which typically include silicas and relatedmaterials, are well known.

However, the conditions within the glassmelting furnace have been knownto cause corrosion of the inner surfaces of the furnace, especially ofthe roof (“crown”) over the glassmaking materials. The most widely usedmaterial for the crown is silica brick for soda-lime-silicate glassfurnaces. Alkali vapors (mostly NaOH and KOH) generated from the glassbatch material in the glassmelting furnace react with silica refractorybrick and faun over time a glassy silicate material on the inner surfaceof the crown. When a sufficient concentration of alkali oxides (mainlyNa₂O and K₂O) accumulates in the glassy silicate layer, the glassymaterial can become fluid enough to drip directly into the molten glassin the furnace or to run along the silica refractory surface and overother refractory surfaces in the furnace and dissolve or dislodge someof the refractory particles which fall into the molten glass. Thiscorrosion is undesirable as it leads to a loss of material in the crown,which eventually leads to expensive repairs or replacement of the crown,and because the corrosion products have been known to fall into theglassmaking materials in the furnace and to cause defects in the glassproduct.

U.S. Pat. No. 3,238,030 and U.S. Pat. No. 3,240,581, teaches that theformation of the silicate layer on the crown surface is to beencouraged, evidently in the belief that the layer prevented othercorrosion products from causing defects in the glass product.

U.S. Pat. No. 3,238,030 teaches that alkali compounds such as sodiumsulfate that form in the glassmelting process should be permitted toreach the material on the crown surface, whereupon the sulfate issubjected to conditions that convert the sulfate to more silicate on thematerial on the crown surface. However, for reasons mentioned above,that silicate layer eventually becomes a source of defects in the glassproduct and a sign of corrosive damage to the crown.

U.S. Pat. No. 3,240,581 teaches the introduction into the glass furnaceatmosphere of additional amounts of alkali (sodium) even beyond theamounts that are already in the glass furnace atmosphere from theglassmaking materials. Here, too, is a teaching that is now understoodto lead to conditions at the crown surface that cause defects in theglass product and damage to the crown.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is a method of operating aglassmelting furnace, comprising

(A) providing a glassmelting furnace having walls defining a chamberthat encloses a bath comprising glassmaking materials, a crown over saidbath, at least one glassmelting oxy-fuel burner through a wall toprovide heat to said bath by combustion in a combustion zone that islocated between said bath and said crown, wherein the atmosphere withinsaid glassmelting furnace contains vapor-phase alkali species and theinner surface of said crown is susceptible to corrosion by reaction withsaid vapor-phase alkali species, and at least one injector through awall of said furnace that is oriented to inject a gaseous streamcomprising water vapor to flow along said inner surface of said crownbetween said combustion zone and said crown, and

(B) combusting fuel and oxidant having an oxygen content higher thanthat of air at said at least one glassmelting burner to provide heat tosaid bath while injecting from said at least one injector a gaseousstream comprising water vapor, and passing said stream comprising watervapor along said inner surface of said crown at a low velocity toestablish and maintain a gaseous layer of along said inner surface inwhich the concentration of water vapor at said inner surface of saidcrown is higher than the concentration of water vapor in the atmosphereat the surface of said bath.

Preferably, the stream comprising water vapor that is passed along theinner surface of the crown contains no alkali.

Another aspect of the present invention is a method of operating aglassmelting furnace, comprising

(A) providing a glassmelting furnace having walls defining a chamberthat encloses a bath comprising glassmaking materials, a crown over saidbath, at least one glassmelting oxy-fuel burner through a wall toprovide heat to said bath by combustion in a combustion zone that islocated between said bath and said crown, wherein the atmosphere withinsaid glassmelting furnace contains vapor-phase alkali species and theinner surface of said crown is susceptible to corrosion by reaction withsaid vapor-phase alkali species, and at least one auxiliary oxy-fuelburner through a wall of said furnace that produces a gaseous stream ofcombustion products comprising water vapor and that is oriented to feedsaid gaseous stream of combustion products to flow along said innersurface of said crown between said combustion zone and said crown, and

(B) combusting fuel and oxidant having an oxygen content higher thanthat of air at said at least one glassmelting burner to provide heat tosaid bath while combusting fuel and oxidant having an oxygen contenthigher than that of air at said at least one auxiliary oxy-fuel burnerat a stoichiometric ratio of at least 1.0 to produce a gaseous stream ofcombustion products comprising water vapor, and passing a streamcomprising said combustion products along said inner surface of saidcrown at a low velocity to establish and maintain a layer of saidcombustion products along said inner surface in which the concentrationof water vapor at said inner surface of said crown is higher than theconcentration of water vapor in the atmosphere at the surface of saidbath.

Preferably, the stream of combustion products passed along the innersurface of the crown contains no alkali.

Another aspect of the present invention is a method of operating aglassmelting furnace, comprising

(A) providing a glassmelting furnace having walls defining a chamberthat encloses a bath comprising glassmaking materials, a crown over saidbath, at least one glassmelting oxy-fuel burner through a wall toprovide heat to said bath by combustion in a combustion zone that islocated between said bath and said crown, wherein the atmosphere withinsaid glassmelting furnace contains vapor-phase alkali species and theinner surface of said crown is susceptible to corrosion by reaction withsaid vapor-phase alkali species, and at least one auxiliary burnerthrough a wall of said furnace that produces a gaseous stream ofcombustion products and that is oriented to feed said gaseous stream ofcombustion products to flow along said inner surface of said crownbetween said combustion zone and said crown, and

(B) combusting fuel and oxidant having an oxygen content higher thanthat of air at said at least one glassmelting burner at a stoichiometricratio greater than 1.0 to provide heat to said bath, while combustingfuel and oxidant at said at least one auxiliary burner at astoichiometric ratio of less than 1.0 to produce a gaseous stream ofcombustion products comprising carbon monoxide and containing no alkali,and passing said gaseous stream of combustion products along said innersurface of said crown to establish and maintain a layer of saidcombustion products flowing along said inner surface which interact withalkali species from said combustion zone which are thereby inhibitedfrom reacting with said inner surface of said crown.

Yet another aspect of the present invention is a method of operating aglassmelting furnace, comprising

(A) providing a glassmelting furnace having walls defining a chamberthat encloses a bath comprising glassmaking materials, a crown over saidbath, and at least one glassmelting oxy-fuel burner through a wall toprovide heat to said bath by combustion in said chamber, wherein theatmosphere within said chamber contains vapor-phase alkali species andthe inner surface of said crown is susceptible to corrosion by reactionwith said vapor-phase alkali species, and

(B) combusting fuel and oxidant having an oxygen content higher thanthat of air at said at least one glassmelting burner at a stoichiometricratio greater than 1.0 to provide heat to said bath, while injectinginto said chamber a gaseous reactant, containing no alkali, which reactswith alkali species in said chamber which are thereby inhibited fromreacting with said inner surface of said crown.

The reactant is preferably injected through one or more glassmeltingburners, and/or through at least one injector through a wall of saidfurnace.

As used herein, “glassmaking materials” comprise any of the followingmaterials, and mixtures thereof: sand (mostly SiO₂), soda ash (mostlyNa₂CO₃), limestone (mostly CaCO₃ and MgCO₃), feldspar, borax (hydratedsodium borate), other oxides, hydroxides and/or silicates of sodium andpotassium, and glass (such as recycled solid pieces of glass) previouslyproduced by melting and solidifying any of the foregoing. Glassmakingmaterials may also include functional additives such as batch oxidizerssuch as salt cake (calcium sulfate, CaSO₄) and/or niter (sodium nitrate,NaNO₃, and/or potassium nitrate, KNO₃).

As used herein, “alkali species” means chemical compounds containingsodium, potassium and/or lithium atoms, including but not limited tosodium hydroxide, potassium hydroxide, products formed by decompositionof sodium hydroxide or potassium hydroxide at temperatures greater than1200° C., and mixtures thereof.

As used herein, “oxy-fuel burner” means a burner through which are fedfuel and oxidant having an oxygen content greater than the oxygencontent of air, and preferably having an oxygen content of at least 50volume percent and more preferably more than 90 volume percent.

As used herein, “oxy-fuel combustion” means combustion of fuel withoxidant having an oxygen content greater than the oxygen content of air,and preferably having an oxygen content of at least 50 volume percentand more preferably more than 90 volume percent.

As used herein, “alkali hydroxide” means material selected from thegroup consisting of sodium hydroxide, potassium hydroxide, and mixturesthereof.

As used herein, “stoichiometric ratio” means the ratio of oxygen presentto the total amount of oxygen that would be necessary to convert fullyall carbon, sulfur and hydrogen present to carbon dioxide, sulfurdioxide, and water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view along the length of a glassmeltingfurnace in which the present invention can be practiced.

FIG. 2 is a top cross-sectional view of a glassmelting furnace in whichthe embodiments of the present invention can be practiced.

FIG. 3 is a cross-sectional view along the length of a glassmeltingfurnace in which another embodiment of the present invention can bepracticed.

FIG. 4 is a cross-sectional view along the length of a glassmeltingfurnace in which another embodiment of the present invention can bepracticed.

FIG. 5 is a cross-sectional view along the length of a glassmeltingfurnace in which another embodiment of the present invention can bepracticed.

FIG. 6 is a cross-sectional view along the length of a glassmeltingfurnace in which another embodiment of the present invention can bepracticed.

DETAILED DESCRIPTION OF THE INVENTION

The invention is carried out in a glassmelting furnace 10 of anyeffective design. Typically the glassmelting furnace has a bottom 12 andwalls 14 which define in the interior of the furnace a chamber forholding a bath 16 of glassmaking materials. Typically, at least aportion of the glassmaking materials in the bath are molten. That is,“glassmelting furnace” in which the present invention can be practicedincludes apparatus in which glassmaking materials are melted, andincludes apparatus (or sections of glassmelting apparatus, sometimesreferred to as conditioning or refining zones) which holdsalready-molten glassmaking materials without necessarily performing anyfurther melting.

The particular shape of the bottom is not critical, although in generalpractice it is preferred that at least a portion of the bottom is planarand is either horizontal or sloped in the direction of the flow of themolten glass through the furnace. All or a portion of the bottom caninstead be curved. The particular shape of the furnace as defined by itswalls is also not critical, so long as the walls are high enough to holdthe desired amount of molten glass and to provide with the crown spaceabove the molten glass in which the combustion can occur that melts theglassmaking materials and keeps them molten.

As seen better in FIG. 2, the furnace 10 also has an entrance 18,typically in a back wall 13 at an end of the furnace through whichglassmaking materials or molten glass flows into the furnace, and anexit 20, typically in a front wall 15 at an end of the furnace oppositefront wall 13 through which molten glass can flow out of the furnace.The furnace will also have side walls 14 and a roof, referred to hereinas the crown 22. There are also one or more flues 24 through whichproducts of the combustion of fuel and oxygen can flow out of theinterior of the furnace. The flue or flues are typically located in backwall 13, or in one or more side walls.

The bottom, sides and crown of the furnace should be made fromrefractory material that can retain its solid structural integrity atthe temperatures to which it will be exposed, i.e. typically 1300° C. to1700° C. Such materials are widely known in the field of construction ofhigh-temperature apparatus. Examples include silica, fused alumina, andAZS.

The inner surface of the crown, i.e. the surface that is in contact withthe furnace atmosphere, may be comprised of the original material ofconstruction of the crown, and in some places may instead comprise alayer of slag that has formed on what was the uncorroded surface of thecrown. Such a slag layer may often be found in furnaces that havealready been in use, particularly when such furnaces have been heated byoxy-fuel combustion. Typically, the slag layer contains silica, alkalioxide, alkaline earth oxide, and compounds thereof, such as containcalcium oxide and/or compounds of calcium oxide with silica and/oralkali oxide. Thus, the present invention can be carried out in furnacesin which the inner surface of the crown comprises corrosion productformed by reaction of the surface with alkali hydroxide, and in furnacesin which the inner surface of the crown does not comprise corrosionproduct formed by reaction of the surface with alkali hydroxide.

Referring again to FIG. 2, the glassmaking furnace 10 is also equippedwith one or more glassmelting burners 26. Preferably, one or more ofglassmelting burners 26 are oxy-fuel burners, that is, burners at whichthe combustion that occurs is oxy-fuel combustion. More preferably, forease of construction, ease of operation and satisfactory control of theoperations, all of the burners 26 are oxy-fuel burners. However, ifdesired, air-fired burners can be present as well.

Each glassmelting burner 26 is arranged so that oxidant and fuel are fedfrom suitable sources outside the furnace, into and through the burnerso that fuel and oxidant emerge from the burner (separately or mixed)and combust in the interior of the furnace and generate heat to melt theglassmaking materials and maintain the glassmelt in the molten state.

The oxidant fed to the oxy-fuel burners is gaseous and should have ahigher oxygen content than air, i.e. higher than 21 volume percent, butthe preferred oxygen content of the oxidant is at least 50 volumepercent and more preferably at least 90 volume percent. Thus,oxygen-enriched air can be employed as the oxidant, as can higher purityoxygen produced on site or purchased from a commercial supplier. Thefuel fed to the burners should contain carbon, hydrocarbons or othercompounds containing hydrogen and/or carbon. Suitable fuels includethose conventionally used for these purposes such as natural gas, fueloil, pulverized coal or petroleum coke. The stoichiometric ratio atthese burners is preferably greater than 1.0, preferably 1.03 to 5; thatis, there is preferably excess oxygen which may be present in thecombustion zone atmosphere. This is attained in conventional manner, byadjusting the flow rate of the oxidant (taking into account its oxygenconcentration) relative to the flow rate of the fuel (taking intoaccount its content of combustible matter).

The glassmelting burners 26 are arranged so that the combustion of thefuel and oxidant fed to the burner occurs in a combustion zone 28 whichincludes flame(s) 29, in the interior of the furnace, above the surfaceof the bath 16 of melting or molten glassmaking materials and belowcrown 22. Typically the burners 26 can be located in one or more wallsof the furnace (by which is meant side walls 14, back wall 13, and/orfront wall 15). Burners 26 can be oriented so that the axes of theflames they support extend inward parallel to the surface of the moltenglass, or so that the flames extend from the burner in a directiontoward the surface of the molten glass. Combinations of differentorientations can also be employed. Preferred examples include providingequal numbers of burners 26 in each of the side walls of a furnace,facing each other or staggered. The burners 26 can all be at the sameheight above the top surface of the bath 16, or the heights can bevaried to provide a different distribution of the heat of combustion.

The glassmelting furnace can also include a section in which there is noglassmelting burner 26 above the bath, wherein the molten glass ismaintained in the molten state by the heat within the chamber.

In the practice of one advantageous aspect of the present invention, oneor more injectors are provided and used as described below. One suchinjector 30 is shown in FIG. 1, but more than one such injector 30 maybe provided and employed depending on the size of the furnace 10. Asshown in FIG. 1, injector 30 is located through a suitably dimensionedhole in a side wall 14. Injector 30 is oriented so that the gaseousstream that it injects from its end 32 flows along the inner surface ofcrown 22 that faces the interior of furnace 10 (as described above, theinner surface of the crown is considered to be the surface that isotherwise in contact with the furnace atmosphere, whether that surfaceis comprised of the original material from which the crown wasconstructed, or is comprised of slag such as is formed upon corrosion byinteraction of the furnace atmosphere with the original material fromwhich the inner surface of the crown is constructed). Each injector 30is located high on a front, back, or side wall, near or at theintersection of the wall with the crown. Each injector 30 should beoriented so that the gaseous stream that emerges from end 32 flowsparallel to the portion of the inner crown surface that is closest tothe end 32. The flow of the gaseous stream from injector 30 is depictedin FIG. 1 as arrow 34. The gaseous stream thus passes between combustionzone 28 and crown 22. Each injector 30, or at least the portions thereofthat are exposed to the high temperatures prevailing within furnace 10,must be made of material that can withstand exposure to suchtemperatures. Ceramic and high-temperature steel are examples ofsuitable material. Preferably, the injector 30 has a discharge end 32which is configured, or contains a nozzle that is configured, so thatthe gaseous stream is discharged in a wide, flat pattern, the better toprovide flow of the stream along more of the inner surface of the crown.

The gaseous stream injected from injector 30 comprises water vapor, i.e.steam. The stream may comprise 100 vol. % water vapor, but it can haveany water vapor concentration less than 100 vol. % such as at least 25vol. 5, at least 50 vol. %, and even at least 75 vol. %, so long as thewater vapor concentration in the layer at the inner surface of the crownis higher than the water vapor concentration in the atmosphere at thesurface of the molten glass bath. Thus, water vapor from a suitablesource 36 such as a steam generator, optionally combined with one ormore other gaseous components represented by source 38, is fed to eachof the one or more injectors 30 employed in the practice of thisinvention.

The stream is emitted from injector 30 as a gaseous stream at a lowvelocity, effective to establish and maintain a layer having the desiredhigh water vapor content as described herein at the inner surface of thecrown and permitting the water vapor in that layer to establish thedesired equilibrium conditions at the inner surface of the crown. Thisis quite different from prior art techniques that direct a stream toflow toward the crown surface and to impinge on that surface or onmaterial that is present on that surface. This is also quite differentfrom prior art techniques employing so-called “purge burners” whicheject high velocity streams along the crown surface to purge or sweeppotentially corrosive gases from reaching the crown surface. That highvelocity minimizes or prevents chemical interaction of the crown surfacewith the streams from the purge burners, whereas the present inventionrelies on interaction of the crown surface with the combustion productsfrom the injector 30 and requires low velocity to help attain thatinteraction.

In the practice of another aspect of the present invention, one or more,up to all, of the injectors 30 described above is an oxy-fuel burner,which will be referred to herein as an auxiliary oxy-fuel burner inorder to distinguish it from oxy-fuel burners 26. Referring to FIG. 3,in which reference numerals that also appear in FIG. 1 has the samemeaning as disclosed above with respect to FIG. 1, reference numeral 130designates one such auxiliary oxy-fuel burner, although more than onesuch burner 130 may be provided and employed depending on the size ofthe furnace 10. Auxiliary oxy-fuel burner 130 is located through asuitably dimensioned hole in a side wall 14. Burner 130 is oriented sothat the gaseous combustion products that it produces by combustion atits end 132 flow along the inner surface of crown 22 that faces theinterior of furnace 10 (as described above, the inner surface of thecrown is considered to be the surface that is otherwise in contact withthe furnace atmosphere, whether that surface is comprised of theoriginal material from which the crown was constructed, or is comprisedof slag such as is formed upon corrosion by interaction of the furnaceatmosphere with the original material from which the inner surface ofthe crown is constructed). Each oxy-fuel burner 130 is located high on afront, back, or side wall, near or at the intersection of the wall withthe crown. Each oxy-fuel burner 130 should be oriented so thatcombustion products as they emerge from end 132 flow parallel to theportion of the inner crown surface that is closest to the end 132. Theflow of the stream of gaseous combustion products from oxy-fuel burner130 is depicted in FIG. 3 as arrow 134. The gaseous combustion productsthus pass between combustion zone 28 and crown 22. Each oxy-fuel burner130, or at least the portions thereof that are exposed to the hightemperatures prevailing within furnace 10, must be made of material thatcan withstand exposure to such temperatures. Ceramic andhigh-temperature steel are examples of suitable material. Preferably,the oxy-fuel burner 130 has a discharge end 132 which is configured, orcontains a nozzle that is configured, so that the stream of combustionproducts is discharged in a wide, flat pattern, the better to provideflow of the combustion products along more of the inner surface of thecrown.

When using auxiliary oxy-fuel burner 130, fuel from fuel source 136 andoxidant from oxidant source 138 are fed to each of the one or moreauxiliary oxy-fuel burners 130 employed in the practice of thisinvention. The fuel and the oxidant combust at discharge end 132,producing a stream of gaseous combustion products that passes along theinner surface of the crown. The fuel fed to each oxy-fuel burner 130 canbe any combustible gaseous or liquid hydrocarbon, preferred examples ofwhich include natural gas and fuel oil. The oxidant fed to this oxy-fuelburner has an oxygen content greater than that of air, preferably atleast 50 vol. %, more preferably at least 90 vol. %. The stoichiometricratio at these burners is preferably greater than 1.0, preferably 1.03to 5. This is attained in conventional manner, by adjusting the flowrate of the oxidant (taking into account its oxygen concentration)relative to the flow rate of the fuel (taking into account its contentof combustible matter).

The stream of combustion products is emitted from burner 130 as agaseous stream at a low velocity, effective to establish and maintain alayer having the desired high water vapor content as described herein atthe inner surface of the crown and permitting the water vapor in thatlayer to establish the desired equilibrium conditions at the innersurface of the crown. This is quite different from prior art techniquesthat direct a stream to flow toward the crown surface and to impinge onthat surface or on material that is present on that surface. This isalso quite different from prior art techniques employing so-called“purge burners” which eject high velocity streams along the crownsurface to purge or sweep potentially corrosive gases from reaching thecrown surface. That high velocity minimizes or prevents chemicalinteraction of the crown surface with the streams from the purgeburners, whereas the present invention relies on interaction of thecrown surface with the combustion products from the oxy-fuel burner orburners 130 and requires low velocity to help attain that interaction.

Practice according to these aspects of the present invention establishesa gaseous layer along the inner surface of the crown that has a watervapor concentration higher than the water vapor concentration of theatmosphere at the surface of the bath of molten glassmaking materials.This difference of water vapor concentrations can be attained in manyways.

One way is to operate the glassmelting burners 26 at stoichiometricconditions at which the stream injected from injector 30 has a higherwater vapor content than the water vapor content of the combustionproducts of the glassmelting burners 26. Yet another way is to operatethe glassmelting furnace so that the combustion products of theglassmelting burners are diluted by ambient atmosphere that enters intothe furnace, and/or by gas or mixtures containing little or no watervapor that are injected into the combustion zone of the furnace. Apreferred way is simply to inject sufficient water vapor (steam) in thestream leaving end 32 so that the stream that flows along the innersurface of the crown has a water vapor concentration high enough to behigher than the water vapor concentration at the surface of the bath ofmolten glass. In this embodiment, the gaseous stream that flows alongthe inner surface of the crown can contain water vapor at aconcentration of at least 50 vol. %, preferably at least 75 vol. %, oreven at least 90 vol. %.

When using one or more oxy-fuel burners 130, one way to attain thedesired difference of water vapor concentrations described above is tooperate the auxiliary oxy-fuel burners 130 and the glassmelting burners26 at different stoichiometric conditions, so that the combustionproducts of the auxiliary burners 130 have a higher water vapor contentthan the combustion products of the glassmelting burners 26. Another wayis to use different fuels, so that the fuel that is combusted atburner(s) 26 has a higher C:H ratio than the fuel that is combusted atburner(s) 130. For instance, the fuel combusted at burner(s) 130 couldcontain hydrogen, or could even be 100% hydrogen, whereas the fuelcombusted at burner(s) 26 could contain amounts up to 100% of coke (suchas petroleum coke) having less hydrogen, or even carbon. A preferred wayis to operate the glassmelting furnace so that the combustion productsof the glassmelting burners are diluted by ambient atmosphere thatenters into the furnace, and/or by gas or mixtures containing little orno water vapor that are injected into the combustion of the furnace. Yetanother way is to inject water vapor (steam) into the stream ofcombustion products leaving end 132, or through the auxiliary burner130, so that the stream that flows along the inner surface of the crowncontains water vapor not produced by combustion at the burner 130 inaddition to water vapor that is produced by the combustion at the burner130. In this embodiment, the gaseous stream that flows along the innersurface of the crown can contain water vapor at a concentration of atleast 50 vol. %, preferably at least 75 vol. %, or even at least 90 vol.%.

Requiring the water vapor concentration at the crown to be higher thanat the bath surface is believed to be surprising, given theunderstanding in this field that higher rates of crown corrosion areassociated with oxy-fuel combustion which is understood to produce morewater vapor in its combustion products compared to the combustionproducts of air-fuel combustion. However, the findings of the presentinvention indicate that the conditions described herein prevent crowncorrosion, and without intending to be bound by any particular theory ofhow this invention works, these findings are consistent with thefollowing.

Under the temperature conditions established by the combustion at theoxy-fuel glassmelting burner or burners 26, wherein the temperaturewithin the furnace is high enough to maintain the glassmaking materialsin a molten condition, the atmosphere within the furnace will containsome vapor-phase alkali hydroxide. The vapor-phase alkali hydroxide isbelieved to form by so-called reactive volatilization in the glassmeltsurface between water vapor in the atmosphere and alkali oxide (e.g.sodium oxide and/or potassium oxide) in the bath. The water vapor ispresent as a product of combustion at the oxy-fuel burner or burners 26,and as a product of vaporization of water contained in batch materialsfed to the furnace. This vapor-phase alkali hydroxide content can bedetermined by measurement or by computer simulation.

For any given temperature and composition at the crown surface, there isa threshold surface concentration of alkali above which crown corrosionwill progress. According to this invention, the alkali concentration inthe crown surface is controlled below that threshold concentration byincreasing the partial pressure of water vapor near the crown surface. Ahigher water vapor concentration will shift the equilibrium vapor phaseconcentration of alkali hydroxide to a higher level for a given alkaliconcentration in the crown surface. The corrosion rate of the crownmaterial by alkali hydroxide is believed to be proportional to thedifference between the partial pressure of the alkali hydroxide in thefurnace atmosphere near the inner crown surface, and the alkalihydroxide's equilibrium vapor pressure on the surface. The partialpressures are in turn proportional to the concentrations. Reducing thisdifference would accordingly reduce the rate of alkali hydroxidetransport to the crown material; reducing this difference to zero wouldeliminate that rate and its concomitant corrosion. And since the alkalihydroxide's equilibrium vapor pressure on the surface is proportional toseveral factors one of which is the partial pressure of water vapor atthat surface, increasing the water vapor concentration at that surfaceincreases the alkali hydroxide's equilibrium vapor pressure at thesurface. Doing so will also tend to decrease the aforementioneddifference in partial pressures of the alkali hydroxide. Accordingly,increasing the water vapor concentration at the inner crown surfaceincreases the equilibrium alkali hydroxide concentration at the innercrown surface, which is proportional to the square root of the watervapor concentration there. The net result is that providing sufficientwater vapor at the inner crown surface to increase the equilibriumalkali hydroxide vapor pressure at the inner crown surface diminishes,or even eliminates, the difference in alkali hydroxide partial pressureswhich is believed to be the driving force otherwise favoring thecorrosive reaction at the inner crown surface. Thus, the presentinvention achieves the desired control of crown corrosion by chemicalinteraction, not simply by dilution.

This result would be achieved by injecting from injector 30 a streamthat has a high enough water vapor content, and by passing this streamalong the inner surface of the crown at a low velocity so that thegaseous stream can flow along the inner crown surface and provide andmaintain the desired high concentration of water vapor at that surface.Suitable velocities for the stream as injected from end 32 would be onthe order of 0.5 feet per second to 50 feet per second, and preferably0.5 feet per second to 20 feet per second.

This result would be achieved by carrying out oxy-fuel combustion offuel and oxidant at burner 130 under conditions that the gaseouscombustion products comprise a high enough water vapor content, and bypassing these combustion products along the inner surface of the crownat a low velocity so that the gaseous products can flow along the innercrown surface and provide and maintain the desired high concentration ofwater vapor at that surface. Suitable velocities for the stream ofcombustion products from the burners 30 would be on the order of 0.5feet per second to 50 feet per second, and preferably 0.5 feet persecond to 20 feet per second.

It can thus also be seen that the method of the present invention isbased on retarding or preventing the progress of corrosion. Therefore,the method is preferably operated on a continuous basis, rather thanintermittently, as intermittent operation would permit corrosion toreoccur, and would permit slag to recommence forming on the crownsurface, and the present invention is based on not permitting either ofthose events from happening.

It should also be noted that in a preferred embodiment of this inventionthe stream that is directed to flow along the inner surface of thecrown, comprising combustion products from the auxiliary oxy-fuelburner, contains no sodium or other alkali. This is quite different fromthose prior art techniques that require contacting the crown surfacewith an alkali-containing stream.

Preferably, the stream of water vapor-containing combustion productsemerging from ends 32 reaches all the way across the crown innersurface, but where burners 30 are provided on both sides of the furnace,each should discharge its stream of combustion products so that actingtogether they cover as much as possible, preferably all, of the innercrown surface. For instance, if burners like burner 30 are positioned onopposing walls, they could discharge their streams of combustionproducts so that each stream reaches to at least the center of thecrown. It is also preferred that the temperature of the stream ofcombustion products from burner or burners 30 is within about 100 F orless of the temperature of the inner surface of the crown, to minimizetemperature stresses.

It has been determined by detailed computer simulation that using aburner such as burner 30 to increase the water vapor concentration atthe inner crown surface would impede or cease corrosion of the crownsurface. In this simulation, the alkali hydroxide was assumed to besodium hydroxide. The conditions and results of that simulation areprovided in Example 1.

Example 1

This Example describes a simulation of the effect of using theaforementioned oxy-fuel burner (referred to here as “the added burner”)to reduce silica crown corrosion of a glassmelting furnace in which theglassmelting heat was provided by oxy-fuel combustion carried out usingoxy-fuel burners. The furnace that was modeled was 10.5 m long and 5.4 mwide and had a nominal design pull of 120 tons per day. The originalfurnace design was not optimized hence heavy corrosion at various partsof the silica crown was observed. Computer models of a glass furnace andof heterogeneous phase equilibriums together with engineeringcalculations were applied for the analysis, and Table 1 tabulates theresults.

Crown corrosion rates were determined at three selected sampling pointsat the centerline of the furnace crown in the furnace length direction.These sampling points were located at various distances away from thebatch charge-end. Lengthwise, the sampling locations were at about 25%,63%, and 85% of the total furnace length, respectively. The baselinecorrosion status, i.e., when the added burner was not used, is listed inTable 1, Part A. Part B of Table 1 lists similar corrosion details atthe selected sampling points when the added burner was operating. Thefollowing sections describe and compare the corrosion details at thethree sampling points.

When the added burner was not operating, gas velocity near the crown atsampling point # 1 was calculated to be 0.43 m/sec. The temperature ofthe furnace gas was calculated to be 1488° C. and to contain 54 wt. % ofwater vapor and 288 ppm of NaOH vapor. The temperature of the crownsurface was calculated to be 1450° C. For a typical silica brick surfacelayer containing 2.46 weight % of CaO and 1.64 wt. % Na₂O, thecalculated equilibrium concentration of NaOH vapor was calculated to be159 ppm on a surface of SiO₂—Na₂O—CaO slag, i.e. the product of silicacrown corrosion, calculated to contain 88% (by weight) of SiO₂. Sincethe calculated concentration of NaOH in the furnace gas (288 ppm) washigher than that of the equilibrium NaOH on the slag surface (159 ppm),there would be a net mass of NaOH being transferred to the silica crown,which would be expected to cause crown corrosion. The rate of this NaOHmass transfer would also be a function of the local Reynolds and Schmidtnumbers. The calculated corrosion rate was 2.55 cm/year at this locationwhen the bulk density of the silica brick was assumed to be at a typicalvalue of 1830 kg/m³.

The added burner, which was assumed to be an oxy-fuel burner, wasinstalled near sampling point #1. This burner was assumed to combustnatural gas in 99 vol. % purity oxygen to produce gaseous combustionproducts into which steam was injected, such that the stream fed alongthe crown surface was assumed to be at a temperature of 1488° C. and tocontain 89% of H₂O. Assuming that the original furnace gas was mixedwith the exhaust of the added burner in equal volumes, the resulting gasmixture would contain 71 wt. % of H₂O and 144 ppm of NaOH vapor. Due tothe increase of the water vapor concentration (from 54% original to71%), the amount of NaOH on the crown surface was calculated to increaseto 183 ppm.

Under these conditions, the calculated gas phase NaOH concentration (144ppm) was lower than the calculated equilibrium NaOH vapor concentration(183 ppm) on the slag surface; therefore, there would be expected a netmass transfer of NaOH from the silica crown. The theoretical crowncorrosion rate would thus reduce to zero and the use of the added burnerwould have prevented silica crown corrosion at sampling point #1.

Table 1 also lists similar details of the corrosion status comparisonsfor sampling point #2. The calculated corrosion rate at sampling point#2 was extremely high at 10.32 cm/year when the added burner was notused. This high rate of silica crown corrosion was attributed to highNaOH concentration in the furnace gas (calculated to be 316 ppm) and thehigh weight percentage of SiO₂ (calculated to be 95 wt. %) in the slagformed. When an added burner with the listed operating characteristics(i.e., exhaust at 1576° C. and 88% H₂O) was assumed to be operating nearthis sampling point, the calculated H₂O concentration in the furnace gasincreased to 73% and correspondingly the calculated NaOH concentrationin the furnace gas decreased to 158 ppm. The calculated equilibrium NaOHconcentration on the slag surface was 183 ppm. Since the calculatedgas-phase NaOH concentration (158 ppm) was lower than the calculatedequilibrium NaOH vapor concentration (183 ppm) on the slag surface,there would be no net mass of NaOH transferred to the silica crown. Thetheoretical crown corrosion rate would also be reduced to zero.

Sampling point #3 was located near the furnace throat-end and beyond thelast oxy-fuel fired burner in the furnace length direction. The lack ofactive burners beyond this point resulted in a very low calculated gasvelocity near the crown of 0.09 m/sec. The calculated Reynolds numberwas 797 indicating a laminar flow regime existed near the crown surface.The calculated crown corrosion rate was 5.14 cm/year despite that thecalculated NaOH concentration in the furnace gas was the highest (393ppm) among the three sampling points. This calculated moderation insilica crown corrosion is believed to be due to the lower NaOH masstransfer coefficient achievable when the furnace gas near the crown isin the laminar flow regime. If an added burner, producing combustionproducts at 1540 C and containing 88 wt. % of H₂O in the burner exhaust,is added and used near sampling point #3, the calculated H₂Oconcentration in the furnace gas increases to 73% and correspondinglythe calculated NaOH concentration in the furnace gas decreases to 196ppm. The calculated equilibrium NaOH concentration on the slag surfacewas 186 ppm. Since the calculated gas-phase NaOH concentration (196 ppm)is slightly higher than the calculated equilibrium NaOH vaporconcentration (186 ppm) on the slag surface, there would be expected anet mass of NaOH transferred to the silica crown, causing crowncorrosion. The calculated corrosion rate was 0.32 cm/year when thisadded burner was assumed to be used. Therefore, the use of the addedburner would be expected to reduce silica crown loss by 94% at samplingpoint #3.

TABLE 1 Sample point #1 Sample point #2 Sample point #3 Distance to backwall (m) 2.6 6.6 8.9 Distance as % of furnace length (%) 25 63 85 Part A-- without purge burner Gas Phase Properties near Crown: Gas velocity atcrown surface (m/sec) 0.43 0.46 0.09 Gas temperature near crown (C.)1488 1576 1540 H2O concentration in gas (%) 54 57 59 NaOH concentrationin gas (ppmv) 288 316 393 Properties of Silica Slag on Crown: Crownsurface temperature (C.) 1450 1535 1520 Equilibrium NaOH concentration(ppmv) 159 163 167 Weight % of SiO2 in slag (%) 88 95 94 CalculatedSilica Crown Corrosion: Reynolds number near crown 4461 4269 797 Schmidtnumber 0.65 0.68 0.69 Corrosion rate (cm/year) 2.55 10.32 5.14 Part B --with purge burner Properties of purge flame Flame temperature (C.) 14881576 1540 H2O concentration in purge flame (%) 89 88 88 Gas PhaseProperties near Crown: purge gas/furnace gas (volume ratio) 1 1 1 Gastemperature near crown (C.) 1488 1576 1540 Gas velocity at crown surface(m/sec) 0.85 0.92 0.17 H2O concentration in gas (%) 71 73 73 NaOHconcentration in gas (ppmv) 144 158 196 Properties of Silica Slag onCrown: Crown surface temperature (C.) 1450 1535 1520 Equilibrium NaOHconcentration (ppmv) 183 183 186 Weight % of SiO2 in slag (%) 88 95 94Calculated Silica Crown Corrosion: Reynolds number near crown 8922 85381595 Schmidt number 0.65 0.68 0.69 Corrosion rate (cm/year) 0 0 0.32

As indicated above, the applicant has discovered other methods foroperating a glassmelting furnace by which corrosion of the crown of theglassmelting furnace is inhibited or lessened.

One method is the aspect mentioned above in which fuel and oxidanthaving an oxygen content higher than that of air are combusted at saidat least one glassmelting burner at a stoichiometric ratio greater than1.0 to provide heat to said bath, while combusting fuel and oxidant atleast one auxiliary burner at a stoichiometric ratio of less than 1.0 toproduce a gaseous stream of combustion products comprising carbonmonoxide and containing no alkali, and passing said gaseous stream ofcombustion products along said inner surface of said crown to establishand maintain a layer of said combustion products flowing along saidinner surface which interact with alkali species from said combustionzone which are thereby inhibited from reacting with said inner surfaceof said crown.

This aspect can be carried out in a glassmelting furnace such as thefurnace illustrated in FIG. 4, in which reference numeral that alsoappear in FIG. 1 have the same meaning as disclosed above with respectto FIG. 1.

This aspect of the invention can be carried out in a glassmeltingfurnace 10 of any effective design. Typically the glassmelting furnacehas a bottom 12 and walls 14 which define in the interior of the furnacea chamber for holding a bath 16 of glassmaking materials. Typically, atleast a portion of the glassmaking materials in the bath are molten.That is, “glassmelting furnace” in which the present invention can bepracticed includes apparatus in which glassmaking materials are melted,and includes apparatus (or sections of glassmelting apparatus, sometimesreferred to as conditioning or refining zones) which holdsalready-molten glassmaking materials without necessarily performing anyfurther melting.

The particular shape of the bottom is not critical, although in generalpractice it is preferred that at least a portion of the bottom is planarand is either horizontal or sloped in the direction of the flow of themolten glass through the furnace. All or a portion of the bottom caninstead be curved. The particular shape of the furnace as defined by itswalls is also not critical, so long as the walls are high enough to holdthe desired amount of molten glass and to provide with the crown spaceabove the molten glass in which the combustion can occur that melts theglassmaking materials and keeps them molten.

As seen better in FIG. 2, the furnace 10 also has an entrance 18,typically in a back wall 13 at an end of the furnace through whichglassmaking materials or molten glass flows into the furnace, and anexit 20, typically in a front wall 15 at an end of the furnace oppositefront wall 13 through which molten glass can flow out of the furnace.The furnace will also have side walls 14 and a roof, referred to hereinas the crown 22. There are also one or more flues 24 through whichproducts of the combustion of fuel and oxygen can flow out of theinterior of the furnace. The flue or flues are typically located in backwall 13, or in one or more side walls.

The bottom, sides and crown of the furnace should be made fromrefractory material that can retain its solid structural integrity atthe temperatures to which it will be exposed, i.e. typically 1300° C. to1700° C. Such materials are widely known in the field of construction ofhigh-temperature apparatus. Examples include silica, fused alumina, andAZS.

The inner surface of the crown, i.e. the surface that is in contact withthe furnace atmosphere, may be comprised of the original material ofconstruction of the crown, and in some places may instead comprise alayer of slag that has formed on what was the uncorroded surface of thecrown. Such a slag layer may often be found in furnaces that havealready been in use, particularly when such furnaces have been heated byoxy-fuel combustion. Typically, the slag layer contains silica, alkalioxide, alkaline earth oxide, and compounds thereof, such as containcalcium oxide and/or compounds of calcium oxide with silica and/oralkali oxide. Thus, the present invention can be carried out in furnacesin which the inner surface of the crown comprises corrosion productformed by reaction of the surface with alkali hydroxide, and in furnacesin which the inner surface of the crown does not comprise corrosionproduct formed by reaction of the surface with alkali hydroxide.

Referring again to FIG. 2, the glassmaking furnace 10 is also equippedwith one or more glassmelting burners 26. Preferably, one or more ofglassmelting burners 26 are oxy-fuel burners, that is, burners at whichthe combustion that occurs is oxy-fuel combustion. More preferably, forease of construction, ease of operation and satisfactory control of theoperations, all of the burners 26 are oxy-fuel burners. However, ifdesired, air-fired burners can be present as well.

Each glassmelting burner 26 is arranged so that oxidant and fuel are fedfrom suitable sources outside the furnace, into and through the burnerso that fuel and oxidant emerge from the burner (separately or mixed)and combust in the interior of the furnace and generate heat to melt theglassmaking materials and maintain the glassmelt in the molten state.

The oxidant fed to the oxy-fuel burners is gaseous and should have ahigher oxygen content than air, i.e. higher than 21 volume percent, butthe preferred oxygen content of the oxidant is at least 50 volumepercent and more preferably at least 90 volume percent. Thus,oxygen-enriched air can be employed as the oxidant, as can higher purityoxygen produced on site or purchased from a commercial supplier. Thefuel fed to the burners should contain carbon, hydrocarbons or othercompounds containing hydrogen and/or carbon. Suitable fuels includethose conventionally used for these purposes such as natural gas, fueloil, pulverized coal or petroleum coke. The stoichiometric ratio atthese burners is preferably greater than 1.0, preferably 1.03 to 5; thatis, there is preferably excess oxygen which may be present in thecombustion zone atmosphere.

The glassmelting burners 26 are arranged so that the combustion of thefuel and oxidant fed to the burner occurs in a combustion zone 28 whichincludes flame(s) 29, in the interior of the furnace, above the surfaceof the bath 16 of melting or molten glassmaking materials and belowcrown 22. Typically the burners 26 can be located in one or more wallsof the furnace (by which is meant side walls 14, back wall 13, and/orfront wall 15). Burners 26 can be oriented so that the axes of theflames they support extend inward parallel to the surface of the moltenglass, or so that the flames extend from the burner in a directiontoward the surface of the molten glass. Combinations of differentorientations can also be employed. Preferred examples include providingequal numbers of burners 26 in each of the side walls of a furnace,facing each other or staggered. The burners 26 can all be at the sameheight above the top surface of the bath 16, or the heights can bevaried to provide a different distribution of the heat of combustion.

The glassmelting furnace can also include a section in which there is noglassmelting burner 26 above the bath, wherein the molten glass ismaintained in the molten state by the heat within the chamber.

In the practice of this aspect of the present invention, at least oneauxiliary burner 230 is provided and used as described below. Eachauxiliary burner 230 can be an oxy-fuel burner, at which oxy-fuelcombustion is carried out as described herein, or an air-fuel burner atwhich fuel is combusted with air. Oxy-fuel burners are preferred. Also,where more than one auxiliary burner 230 is employed, some may beoxy-fuel burners while others are air-fuel burners.

One such auxiliary burner 230 is shown in FIG. 4, but more than one suchburner 230 may be provided and employed depending on the size of thefurnace 10. As shown in FIG. 4, auxiliary burner 230 is located througha suitably dimensioned hole in a side wall 14. Burner 230 is oriented sothat the gaseous combustion products that it produces by combustion atits end 232 flow along the inner surface of crown 22 that faces theinterior of furnace 10 (as described above, the inner surface of thecrown is considered to be the surface that is otherwise in contact withthe furnace atmosphere, whether that surface is comprised of theoriginal material from which the crown was constructed, or is comprisedof slag such as is formed upon corrosion by interaction of the furnaceatmosphere with the original material from which the inner surface ofthe crown is constructed). Each auxiliary burner 230 is located high ona front, back, or side wall, near or at the intersection of the wallwith the crown. Each auxiliary burner 230 should be oriented so thatcombustion products as they emerge from end 232 flow parallel to theportion of the inner crown surface that is closest to the end 232. Theflow of the stream of gaseous combustion products from burner 230 isdepicted in FIG. 4 as arrow 234. The gaseous combustion products thuspass between combustion zone 28 and crown 22. Each auxiliary burner 230,or at least the portions thereof that are exposed to the hightemperatures prevailing within furnace 10, must be made of material thatcan withstand exposure to such temperatures. Ceramic andhigh-temperature steel are examples of suitable material. Preferably,burner 230 has a discharge end 232 which is configured, or contains anozzle that is configured, so that the stream of combustion products isdischarged in a wide, flat pattern, the better to provide flow of thecombustion products along more of the inner surface of the crown.

Fuel from fuel source 236 and oxidant from oxidant source 238 are fed toeach auxiliary burner 230 employed in the practice of this aspect of theinvention. The fuel and the oxidant combust at discharge end 232,producing a stream of gaseous combustion products that passes along theinner surface of the crown. The fuel fed to each burner 230 can be anycombustible gaseous or liquid hydrocarbon, preferred examples of whichinclude natural gas and fuel oil. The oxidant fed to an oxy-fuelauxiliary burner has an oxygen content greater than that of air,preferably at least 50 vol. %, more preferably at least 90 vol. %. Theoxidant fed to an air-fuel burner is air. The stoichiometric ratio ateach auxiliary burner 230 is less than 1.0, preferably 0.75 to 0.95.This is attained in conventional manner, by adjusting the flow rate ofthe oxidant (taking into account its oxygen concentration) relative tothe flow rate of the fuel (taking into account its content ofcombustible matter).

The combustion at a stoichiometric ratio less than 1.0 produces agaseous stream of combustion products that includes products ofincomplete combustion, including carbon monoxide. The incompletelyreacted products establish that the gaseous stream is a reducingatmosphere. This stream of combustion products is emitted from burner 30as a gaseous stream at a low velocity, effective to establish andmaintain a gaseous layer at the inner surface of the crown andestablishing the reducing conditions in that layer between thecombustion zone and the inner surface of the crown. This is quitedifferent from prior art techniques that direct a stream to flow towardthe crown surface and to impinge on that surface or on material that ispresent on that surface. This is also quite different from prior arttechniques employing so-called “purge burners” which eject high velocitystreams along the crown surface to purge or sweep potentially corrosivegases from reaching the crown surface. That high velocity minimizes orprevents chemical interaction of reactants with the streams from thepurge burners, whereas the present invention relies on interaction ofalkali species in the combustion zone atmosphere with the combustionproducts from the burner or burners 30 and requires low velocity to helpattain that interaction.

The practice according to this aspect of the present inventionestablishes a gaseous layer along the inner surface of the crown that isa reducing atmosphere. The reducing atmosphere should contain asufficient concentration of oxidizable species to interact with alkalispecies from the combustion zone and inhibit or even completely preventalkali species from reaching and reacting with the inner surface of thecrown. This interaction in the reducing atmosphere that is establishedby the fuel-rich combustion at the at least one burner 230 is believedto reduce oxidic alkali species in the atmosphere to fully reducedmetallic sodium and potassium. These, in turn, pass to other areas ofthe glassmelting furnace and to a flue of the furnace, where theyencounter gases usually containing oxygen (in the products of thecombustion at the glassmelting burners 26) and sulfur dioxide (producedfrom the glassmelt). These gases react with the metallic sodium andpotassium to form alkali oxides and sulfates which exit the glassmeltingfurnace in the flue gas rather than becoming deposited onto the crown.

Requiring the formation of a layer of a gaseous reducing atmosphere atthe crown surface is believed to be surprising, given the teachings inthe aforementioned U.S. Pat. No. 3,238,030 and U.S. Pat. No. 3,240,581which use a reducing atmosphere, and which even add extra alkali, toform ever-increasing amounts of alkali silicate onto the crown surface.However, the present invention differs significantly in that it uses areducing atmosphere to impede and even prevent formation of alkalicompounds onto the crown surface.

The method of the present invention is expected to inhibit and evenprevent formation of alkali compounds onto the crown surface by removinga reactant necessary for that formation. Under the temperatureconditions established by the combustion at the oxy-fuel glassmeltingburner or burners 26, wherein the temperature within the furnace is highenough to maintain the glassmaking materials in a molten condition, theatmosphere within the furnace will contain some vapor-phase alkalispecies (such as alkali hydroxide, and referred to here as such). Thevapor-phase alkali hydroxide is believed to form by so-called reactivevolatilization in the glassmelt surface between water vapor in theatmosphere and alkali oxide (e.g. sodium oxide and/or potassium oxide)in the bath. The water vapor is present as a product of combustion atthe oxy-fuel burner or burners 26, and as a product of vaporization ofwater contained in batch materials fed to the furnace. This vapor-phasealkali hydroxide content can be determined by measurement or by computersimulation.

The corrosive reaction of the crown material with alkali hydroxide (orother alkali species) is believed to require the alkali species to reactwith that surface. The method of the present invention inhibits theformation of alkali compounds in that surface by shifting the chemicalequilibrium conditions between alkali species in the furnace atmosphereand the surface of the crown material. Thus, the present inventionachieves the desired inhibition of crown corrosion by chemicalinteraction, not simply by dilution.

This result would be achieved by carrying out fuel-rich combustion offuel and oxidant at burner 230 under conditions such that the gaseouscombustion products form a reducing atmosphere, and by passing thesecombustion products along the inner surface of the crown at a lowvelocity so that the gaseous products can flow along the inner crownsurface and provide and maintain the desired reducing atmosphere at thatsurface. Suitable velocities for the stream of combustion products fromthe burner 30 would be on the order of 0.5 feet per second to 50 feetper second, and preferably 0.5 feet per second to 20 feet per second.

It can thus also be seen that the method of the present invention isbased on retarding or preventing the progress of corrosion. Therefore,the method is operated on a continuous basis, rather thanintermittently, as intermittent operation would permit corrosion toreoccur, and would permit slag or reactive precursors of slag (such assodium sulfate or sodium sulfite) to recommence forming on the crownsurface, and the present invention is based on not permitting any ofthose events from happening.

It should also be noted that in this aspect of the invention the streamthat is directed to flow along the inner surface of the crown,comprising combustion products from the auxiliary burner, contains nosodium or other alkali. This is quite different from those prior arttechniques that require contacting the crown surface with analkali-containing stream.

Preferably, the stream of combustion products emerging from ends 232reaches all the way across the crown inner surface, but where burners230 are provided on both sides of the furnace, each should discharge itsstream of combustion products so that acting together they cover as muchas possible, preferably all, of the inner crown surface. For instance,if burners like burner 230 are positioned on opposing walls, they coulddischarge their streams of combustion products so that each streamreaches to at least the center of the crown. It is also preferred thatthe temperature of the stream of combustion products from burner orburners 230 is within about 100 F or less of the temperature of theinner surface of the crown, to minimize temperature stresses.

The applicant has discovered yet another method of operating aglassmelting furnace by which corrosion of the crown of the glassmeltingfurnace is inhibited or lessened. This method is the aspect mentionedabove in which fuel and oxidant having an oxygen content higher thanthat of air are combusted at said at least one glassmelting burner at astoichiometric ratio greater than 1.0 to provide heat to said bath,while injecting into said chamber a gaseous reactant, containing noalkali, which reacts with alkali species in said chamber which arethereby inhibited from reacting with said inner surface of said crown.

This aspect can be carried out in glassmelting furnace such as thefurnace illustrated in FIGS. 5 and 6, in which reference numerals thatalso appear in FIG. 1 have the same meaning as disclosed above withrespect to FIG. 1.

This aspect of the invention can be carried out in a glassmeltingfurnace 10 of any effective design. Typically the glassmelting furnacehas a bottom 12 and walls 14 which define in the interior of the furnacea chamber for holding a bath 16 of glassmaking materials. Typically, atleast a portion of the glassmaking materials in the bath are molten.That is, “glassmelting furnace” in which the present invention can bepracticed includes apparatus in which glassmaking materials are melted,and includes apparatus (or sections of glassmelting apparatus, sometimesreferred to as conditioning or refining zones) which holdsalready-molten glassmaking materials without necessarily performing anyfurther melting.

The particular shape of the bottom is not critical, although in generalpractice it is preferred that at least a portion of the bottom is planarand is either horizontal or sloped in the direction of the flow of themolten glass through the furnace. All or a portion of the bottom caninstead be curved. The particular shape of the furnace as defined by itswalls is also not critical, so long as the walls are high enough to holdthe desired amount of molten glass and to provide with the crown spaceabove the molten glass in which the combustion can occur that melts theglassmaking materials and keeps them molten.

As seen better in FIG. 2, the furnace 10 also has an entrance 18,typically in a back wall 13 at an end of the furnace through whichglassmaking materials or molten glass flows into the furnace, and anexit 20, typically in a front wall 15 at an end of the furnace oppositefront wall 13 through which molten glass can flow out of the furnace.The furnace will also have side walls 14 and a roof, referred to hereinas the crown 22. There are also one or more flues 24 through whichproducts of the combustion of fuel and oxygen can flow out of theinterior of the furnace. The flue or flues are typically located in backwall 13, or in one or more side walls.

The bottom, sides and crown of the furnace should be made fromrefractory material that can retain its solid structural integrity atthe temperatures to which it will be exposed, i.e. typically 1300° C. to1700° C. Such materials are widely known in the field of construction ofhigh-temperature apparatus. Examples include silica, fused alumina, andAZS.

The inner surface of the crown, i.e. the surface that is in contact withthe furnace atmosphere, may be comprised of the original material ofconstruction of the crown, and in some places may instead comprise alayer of slag that has formed on what was the uncorroded surface of thecrown. Such a slag layer may often be found in furnaces that havealready been in use, particularly when such furnaces have been heated byoxy-fuel combustion. Typically, the slag layer contains silica, alkalioxide, alkaline earth oxide, and compounds thereof, such as containcalcium oxide and/or compounds of calcium oxide with silica and/oralkali oxide. Thus, the present invention can be carried out in furnacesin which the inner surface of the crown comprises corrosion productformed by reaction of the surface with alkali hydroxide, and in furnacesin which the inner surface of the crown does not comprise corrosionproduct formed by reaction of the surface with alkali hydroxide.

Referring again to FIG. 2, the glassmaking furnace 10 is also equippedwith one or more glassmelting burners 26. Preferably, one or more ofglassmelting burners 26 are oxy-fuel burners, that is, burners at whichthe combustion that occurs is oxy-fuel combustion. More preferably, forease of construction, ease of operation and satisfactory control of theoperations, all of the burners 26 are oxy-fuel burners. However, ifdesired, air-fired burners can be present as well.

Each glassmelting burner 26 is arranged so that oxidant and fuel are fedfrom suitable sources outside the furnace, into and through the burnerso that fuel and oxidant emerge from the burner (separately or mixed)and combust in the interior of the furnace and generate heat to melt theglassmaking materials and maintain the glassmelt in the molten state.

The oxidant fed to the oxy-fuel burners is gaseous and should have ahigher oxygen content than air, i.e. higher than 21 volume percent, butthe preferred oxygen content of the oxidant is at least 50 volumepercent and more preferably at least 90 volume percent. Thus,oxygen-enriched air can be employed as the oxidant, as can higher purityoxygen produced on site or purchased from a commercial supplier. Thefuel fed to the burners should contain carbon, hydrocarbons or othercompounds containing hydrogen and/or carbon. Suitable fuels includethose conventionally used for these purposes such as natural gas, fueloil, pulverized coal or petroleum coke. The stoichiometric ratio atthese burners is preferably greater than 1.0, preferably 1.03 to 5; thatis, there is preferably excess oxygen which may be present in thecombustion zone atmosphere. This is attained in conventional manner, byadjusting the flow rate of the oxidant (taking into account its oxygenconcentration) relative to the flow rate of the fuel (taking intoaccount its content of combustible matter).

The glassmelting burners 26 are arranged so that the combustion of thefuel and oxidant fed to the burner occurs in a combustion zone 28 whichincludes flame(s) 29, in the interior of the furnace, above the surfaceof the bath 16 of melting or molten glassmaking materials and belowcrown 22. Typically the burners 26 can be located in one or more wallsof the furnace (by which is meant side walls 14, back wall 13, and/orfront wall 15). Burners 26 can be oriented so that the axes of theflames they support extend inward parallel to the surface of the moltenglass, or so that the flames extend from the burner in a directiontoward the surface of the molten glass. Combinations of differentorientations can also be employed. Preferred examples include providingequal numbers of burners 26 in each of the side walls of a furnace,facing each other or staggered. The burners 26 can all be at the sameheight above the top surface of the bath 16, or the heights can bevaried to provide a different distribution of the heat of combustion.

The glassmelting furnace can also include a section in which there is noglassmelting burner 26 above the bath, wherein the molten glass ismaintained in the molten state by the heat within the chamber.

The practice according to this aspect of the present invention injectsinto the furnace atmosphere a reactant which reacts with alkali speciesin that atmosphere to form a reaction product that is less reactive,than the alkali species, with the material of the crown surface.Preferably also, the reaction product is also volatile enough that itstays in the gas phase and leaves the furnace in the flue gas. Theamount of the reactant injected should provide a sufficientconcentration of the reactant to react with alkali species in thecombustion zone and prevent alkali species from reaching and interactingwith the inner surface of the crown.

A sufficient amount of reactant should be injected to react with alkalispecies in the chamber and inhibit or even completely prevent alkalispecies from reaching and reacting with the inner surface of the crown.This reaction with the alkali species forms a reaction product that isstable and volatile at the temperatures encountered in the chamber. Thereaction product, in turn, passes to other areas of the glassmeltingfurnace and to a flue of the furnace, where it exits the furnace in theflue gas rather than interacting with the crown such as by reacting withmaterial on the crown surface.

Suitable reactants need to be able to react with alkali species at thetemperatures, generally above 1000° C., encountered within aglassmelting furnace. The reactant must also react with alkali speciesat such temperatures to form a product that is less reactive, orunreactive, with the material at the surface of the crown.

A preferred reactant is B(OH)₃. This is believed to react with alkalispecies in the furnace atmosphere to form NaBO₂ and KBO₂ which thenleave the furnace in the flue gas, rather than reacting with the crownsurface.

One way to inject the reactant into the furnace chamber is to inject itat one or more of the glassmelting burners 26, either through a suitableport in the burner or premixed with oxidant and/or fuel that is fed tothe burner. In this embodiment, a product comprising the reactant to beinjected into the furnace is fed from source 336 to burner 26. Thereactant can be provided in this embodiment as a solution or suspensionin an appropriate solvent or vehicle that is inert to the reactant, orthe reactant can be provided in a form without solvent or vehicle.Depending on the amount of the reactant to be injected, and on itsreactivity, providing it in a mixture with oxidant or fuel and withoutany liquid solvent or vehicle is generally preferred to facilitatedispensing of controlled amounts of the reactant, without having toadjust the feeds of fuel and oxidant to the burner to provide heat ofvaporization.

An alternate embodiment of the present invention is illustrated in FIG.6. In this embodiment, at least one injector is provided and used asdescribed below. One such injector 330 is shown in FIG. 6, but more thanone such injector 330 may be provided and employed depending on the sizeof the furnace 10. As shown in FIG. 6, injector 330 is located through asuitably dimensioned hole in a side wall 14. Instead, or in addition,injectors of this type can be provided in front and/or back walls aswell. Each injector 330 is oriented so that the stream that it injectspasses into the chamber in furnace 10 between the top surface of bath 16and the crown 22. Each injector 330, or at least the portions thereofthat are exposed to the high temperatures prevailing within furnace 10,must be made of material that can withstand exposure to suchtemperatures. Ceramic and high-temperature steel are examples ofsuitable material. Preferably, each injector 330 has a discharge end 332which is configured, or contains a nozzle that is configured, so thatthe stream is discharged in a wide, flat pattern, the better to provideincreased contact with the other constituents of the furnace atmosphere.

The product to be injected into the furnace, comprising the reactant, isfed from source 338 to injector 330. The reactant can be provided inthis embodiment as a solution or suspension in an appropriate solvent orvehicle that is inert to the reactant, or the reactant can be providedin a form without solvent or vehicle. Depending on the amount of thereactant to be injected, and on its reactivity, providing it in amixture with solvent or vehicle is generally preferred to facilitatedispensing of controlled amounts of the reactant.

This method of the present invention is expected to inhibit and evenprevent formation of alkali compounds onto the crown surface by removinga reactant necessary for that formation. Under the temperatureconditions established by the combustion at the oxy-fuel glassmeltingburner or burners 26, wherein the temperature within the furnace is highenough to maintain the glassmaking materials in a molten condition, theatmosphere within the furnace will contain some vapor-phase alkalispecies (such as alkali hydroxide, and referred to here as such). Thevapor-phase alkali hydroxide is believed to form by so-called reactivevolatilization in the glassmelt surface between water vapor in theatmosphere and alkali oxide (e.g. sodium oxide and/or potassium oxide)in the bath. The water vapor is present as a product of combustion atthe oxy-fuel burner or burners 26, and as a product of vaporization ofwater contained in batch materials fed to the furnace. This vapor-phasealkali hydroxide content can be determined by measurement or by computersimulation.

The corrosive reaction of the crown material with alkali hydroxide (orother alkali species) is believed to require the alkali species to reachthat surface. The method of the present invention removes that alkalispecies before it reaches that surface. Thus, the present inventionachieves the desired control of crown corrosion by chemical interaction,not simply by dilution.

It can thus also be seen that the method of the present invention isbased on retarding or preventing the progress of corrosion. Therefore,the method is operated on a continuous basis, rather thanintermittently, as intermittent operation would permit corrosion toreoccur, and would permit slag or reactive precursors of slag (such assodium sulfate or sodium sulfite) to recommence forming on the crownsurface, and the present invention is based on not permitting any ofthose events from happening.

It should also be noted that in this invention the reactant that isinjected into the furnace atmosphere contains no sodium or other alkali.This is quite different from those prior art techniques that requirecontacting the crown surface with an alkali-containing stream.

Preferably, the reactant is injected in a wide pattern, the better toprovide the opportunity for the reactant to encounter alkali species inthe furnace atmosphere. Also, the interaction of reactant and alkalispecies would be improved with increasing distance of penetration of theflow of the reactant stream into the furnace chamber from the point orpoints from which it is injected.

1. A method of operating a glassmelting furnace, comprising (A)providing a glassmelting furnace having walls defining a chamber thatencloses a bath comprising glassmaking materials, a crown over saidbath, at least one glassmelting oxy-fuel burner through a wall toprovide heat to said bath by combustion in a combustion zone that islocated between said bath and said crown, wherein the atmosphere withinsaid glassmelting furnace contains vapor-phase alkali species and theinner surface of said crown is susceptible to corrosion by reaction withsaid vapor-phase alkali species, and at least one auxiliary burnerthrough a wall of said furnace that produces a gaseous stream ofcombustion products and that is oriented to feed said gaseous stream ofcombustion products to flow along said inner surface of said crownbetween said combustion zone and said crown, and (B) combusting fuel andoxidant having an oxygen content higher than that of air at said atleast one glassmelting burner at a stoichiometric ratio greater than 1.0to provide heat to said bath, while combusting fuel and oxidant at saidat least one auxiliary burner at a stoichiometric ratio of less than 1.0to produce a gaseous stream of combustion products comprising carbonmonoxide and containing no alkali, and passing said gaseous stream ofcombustion products along said inner surface of said crown to establishand maintain a layer of said combustion products flowing along saidinner surface which interact with alkali species from said combustionzone which are thereby inhibited from reacting with said inner surfaceof said crown.
 2. A method according to claim 1 wherein said stream ofcombustion products is passed from said at least one auxiliary burner ata velocity of 0.5 feet per second to 50 feet per second.
 3. A methodaccording to claim 1 wherein said stream of combustion products ispassed from said at least one auxiliary burner at a velocity of 0.5 feetper second to 20 feet per second.
 4. A method according to claim 1wherein oxidant and fuel are combusted at said at least one auxiliaryburner at a stoichiometric ratio of 0.75 to 0.95.